To promote growth, vigor and optimum flowering, iris clumps may be raised and divided every three years or so. Dig up the rhizomes carefully to avoid damage to rhizomes and their roots. Examine them for the presence of worm-like insects called iris borers, which may seriously damage or destroy the plant. If they are found, remove them, cut out the affected tissue and dust with a garden insecticide, such as Sevin, before replanting. Select sound rhizomes with two or more growing points. Rhizomes may be cut apart with a sharp knife, or snapped apart by hand. Be sure to preserve as many rhizome roots as possible. The best time to divide iris is in mid-summer while the plants are dormant. Late July through mid August is preferred.